The present invention is related to processes for dewatering and/or drying a variety of materials. More particularly, the present invention is concerned with dewatering and/or drying of various material using oscillatory flow-reversing gaseous media.
Pulse combustion technology is a known and viable commercial method of enhancing heat and mass transfer in thermal processes. Commercial applications include industrial and home heating systems, boilers, coal gassification, spray drying, and hazardous waste incineration. For example, the following U.S. Patents disclose several industrial applications of pulse combustion: U.S. Pat. No. 5,059,404, issued Oct. 22, 1991 to Mansour et al.; U.S. Pat. No. 5,133,297, issued Jul. 28, 1992 to Mansour; U.S. Pat. No. 5,197,399, issued Mar. 30, 1993 to Mansour; U.S. Pat. No. 5,205,728, issued Apr. 27, 1993 to Mansour; U.S. Pat. No. 5,211,704, issued May 18, 1993 to Mansour; U.S. Pat. No. 5,255,634, issued Oct. 26, 1993 to Mansour; U.S. Pat. No. 5,306,481, issued Apr. 26, 1994 to Mansour et al.; U.S. Pat. No. 5,353,721, issued Oct. 11, 1994 to Mansour et al.; and U.S. Pat. No. 5,366,371, issued Nov. 22, 1994 to Mansour et al., the disclosures of which patents are incorporated by reference herein for the purpose of describing pulse combustion. An article entitled xe2x80x9cPulse Combustion: Impinging Jet Heat Transfer Enhancementxe2x80x9d by P. A. Eibeck et al, and published in Combustion Science and Technology, 1993, Vol. 94, pp. 147-165, describes a method of convective heat transfer enhancement, involving the use of pulse combustor to generate a transient jet that impinges on a flat plate. The article reports enhancements in convective heat transfer of a factor of up to 2.5 compared to a steady-flow impingement.
It is believed that the oscillatory flow-reversing impingement can also provide significant increase in heat and mass transfer in a variety of dewatering and/or drying processes. In particular, it is believed that the oscillatory flow-reversing impingement can provide significant benefits with respect to increasing machine rates in processes using moving conveyer belts for supporting the material being dewatered or dried. In addition, it is believed that the oscillatory flow-reversing impingement may enable one to achieve a substantially uniform drying of the differential-density materials or materials having a non-uniform thickness. It is now also believed that the oscillatory flow-reversing impingement may be successfully applied to dewatering and/or drying of materials, alone or in combination with other water-removing processes, such as through-air drying, steady-flow impingement drying, infra red drying, microwave drying, and drying-cylinder drying where applicable.
Examples of the materials that could be subjected to the impingement flow-reversing drying/dewatering in accordance with the present invention include, without limitation: papers, textiles, plastics, agricultural and food products, biotechnology products, pharmaceutical products, and building materials. The suitable materials may be in either continuous form (for example: plastic, webs), or discontinuous form (for example: sand, granular materials, pellets).
Accordingly, the present invention provides a process and an apparatus for removing water or other liquids from a variety of materials, using the oscillatory flow-reversing impingement gas. The present invention also provides an apparatus comprising a gas-distributing system allowing one to effectively control the distribution of the oscillatory flow-reversing gaseous media (such as air or gas) throughout the surface of the material being dewatered or dried. The present invention provide a gas-distributing system that creates a controlled application (for example, a substantially uniform application) of the oscillatory flow-reversing air or gas onto the material being dewatered or dried.
The present invention provides a novel process and an apparatus for removing water or other liquids from a variety of materials, such as, for example, papers, textiles, plastics, agricultural, biotechnology, food products, pharmaceutical products, and building materials, by using oscillatory flow-reversing air or gas as an impinging medium. The material to be dewatered may have a starting moisture content in a broad range, from about 1% to about 99%.
In its process aspect, the present invention comprises the following steps: providing a material to be dewatered or dried; providing an oscillatory flow-reversing impingement gaseous media (gas or air, or any combination thereof) having a predetermined frequency; providing a gas-distributing system terminating with at least one discharge outlet and designed to deliver the oscillatory flow-reversing impingement gaseous media onto a predetermined portion of the material to be dewatered; and impinging the oscillatory flow-reversing gaseous media onto the material through the gas-distributing system, thereby removing moisture from the material. The oscillatory flow-reversing gaseous media may beneficially be impinged onto the material to be dewatered or dried in a predetermined pattern defining an impingement area of the material.
A water-removing apparatus of the present invention has a machine direction and a cross-machine direction perpendicular to the machine direction. The apparatus of the present invention comprises: a support designed to receive thereon a material to be dewatered or dried and to carry it in the machine direction; at least one pulse generator designed to produce oscillatory flow-reversing air or gas; and at least one gas-distributing system in fluid communication with the pulse generator for delivering the oscillatory flow-reversing air or gas to a predetermined portion of the material to be dewatered or dried. The gas-distributing system terminates with at least one discharge outlet juxtaposed with the support (or with the material when the material is disposed on the support). The support and the at least one discharge outlet form an impingement region therebetween. The impingement region is defined by an impingement distance xe2x80x9cZxe2x80x9d formed between the at least one discharge outlet and the support. In the embodiments of the apparatus comprising a plurality of discharge outlets, the discharge outlets are disposed such as to form a predetermined pattern defining an impingement area xe2x80x9cE.xe2x80x9d The oscillatory flow-reversing gas may be impinged onto the material to provide a substantially even distribution of the gas throughout the impingement area. Alternatively, the oscillatory gas may be impinged onto the material to provide an uneven distribution of the gas throughout the impingement area, thereby allowing control of moisture profiles throughout the surface of the material to be dewatered or dried.
According to the present invention, the pulse generator is a device which is designed to produce oscillatory flow-reversing air or gas having a cyclical velocity/momentum component and a mean velocity/momentum component. A cyclical pressure generated by the pulse generator is converted to a cyclical movement/velocity of large amplitude, comprising negative cycles alternating with positive cycles, the positive cycles having greater momentum and cyclical velocity relative to the negative cycles.
In one embodiment, the pulse generator comprises a pulse combustor, generally comprising a combustion chamber, an air inlet, a fuel inlet, and a resonance tube. The tube operates as a resonator generating standing acoustic waves. The resonance tube is in further fluid communication with a gas-distributing system. As used herein, the term xe2x80x9cgas-distributing systemxe2x80x9d defines a combination of tubes, tailpipes, blow boxes, etc., designed to provide an enclosed path for the oscillatory flow-reversing air or gas produced by the pulse generator, and to deliver the oscillatory flow-reversing air or gas to a pre-determined impingement region (defined herein above), where the oscillatory flow-reversing air or gas is impinged onto the material to be dewatered or dried, thereby removing water therefrom. The gas-distributing system is designed such as to minimize, and preferably avoid altogether, disruptive interference which may adversely affect a desired mode of operation of the pulse combustor or oscillatory characteristics of the flow-reversing gas generated by the pulse combustor. The gas-distributing system delivers the flow-reversing impingement air or gas onto the material to be dewatered or dried through at least one discharge outlet, or nozzle.
The frequency of the oscillatory flow-reversing impingement air or gas is in a range of from about 15 Hz to about 3,000 Hz, more specifically from about 15 Hz to about 1,500 Hz, still more specifically from 15 Hz to 1,000 Hz, and still more specifically from 15 Hz to 500 Hz, depending on a type of the pulse generator and/or desired characteristics of the water-removing process. If the pulse generator comprises the pulse combustor, the frequency may be chosen from about 15 Hz to about 500 Hz. If the pulse generator comprises a rotary valve pulse generator, the frequency may be chosen from about 15 Hz to about 1,500 Hz, more specifically from about 15 Hz to about 500 Hz, and still more specifically from about 15 Hz to about 250 Hz.
A Helmholtz-type resonator may beneficially be used in the pulse generator of the present invention. Typically, the Helmholtz-type pulse generator may be tuned to achieve a desired pulse frequency. In the pulse combustor, the temperature of the oscillatory gas at the exit from the discharge outlets is from about 500xc2x0 F. to about 2500xc2x0 F.
Another embodiment of the pulse generator comprises an infrasonic device. The infrasonic device comprises a resonance chamber in fluid communication with an air inlet through a pulsator. The pulsator generates an oscillating air having infrasound (low frequency) pressure which then is amplified in the resonance chamber and in the resonance tube. The infrasonic device""s frequency of the oscillating flow-reversing air is from 15 Hz to 100 Hz. If desired, the apparatus comprising the infrasonic device may have a means for heating the oscillatory flow-reversing air generated by the infrasonic device. Other embodiments of the pulse generator include, without limitation, solenoid valves, fluidic valves, rotary valves, butterfly valves, vibrating mechanical elements, rotating lobes, slot jets, edge jets, and pizeo electric elements. For a rotary valve pulse generator, for example, a broad temperature range is from ambient to 2500xc2x0 F.
The oscillatory flow-reversing impingement air or gas has two components: a mean component characterized by a mean velocity and a corresponding mean momentum; and an oscillatory, or cyclical, component characterized by a cyclical velocity and a corresponding cyclical momentum. The oscillatory cycles during which the combustion gas moves xe2x80x9cforwardxe2x80x9d from the combustion chamber, and into, through, and from the gas-distributing system are positive cycles; and the oscillatory cycles during which a back-flow of the impingement gas occurs are negative cycles. An average amplitude of the positive cycles is a positive amplitude, and an average amplitude of the negative cycles is a negative amplitude. During the positive cycles, the impingement gas has a positive velocity directed in a positive direction towards the material to be dewatered or dried disposed on the support; and during the negative cycles, the impingement gas has a negative velocity directed in a negative direction. The positive direction is opposite to the negative direction, and the positive velocity is opposite to the negative velocity. The positive velocity component is greater than the negative velocity component, and the mean velocity has the positive direction.
The pulse combustor produces an intense acoustic pressure, typically in the order of 160-190 dB, inside the combustion chamber. This acoustic pressure reaches its maximum level in the combustion chamber. Due to the open end of the resonance tube, the acoustic pressure is reduced to atmospheric at the exit of the resonance tube. This drop in the acoustic pressure results in a progressive increase in cyclical velocity which reaches its maximum at the exit of the resonance tube. It is beneficial to have the Helmholtz-type pulse generator in which the acoustic pressure is minimal at the exit of the resonance tubexe2x80x94in order to achieve a maximal cyclical velocity in the exhaust flow of oscillatory impingement gases. The decreasing acoustic pressure beneficially reduces noise typically associated with sonically enhanced processes of the prior art.
At the exit of the gas-distributing system, the cyclical velocity, ranging from about 1,000 ft/min to about 50,000 ft/min, and more specifically from about 2,500 ft/min to about 50,000 ft/min, is calculated based on the measured acoustic pressure in the combustion chamber. The more specific cyclical velocity is from about 5,000 ft/min to about 50,000 ft/min. The mean velocity is from about 1,000 ft/min to about 25,000 ft/min, more specifically from about 2,500 ft/min to about 25,000 ft/min, and still more specifically from about 5,000 to about 25,000 ft/min.
In order to achieve the desired water-removal rates, the oscillatory flow-reversing impingement gas should preferably form an oscillatory xe2x80x9cflow fieldxe2x80x9d substantially uniformly contacting the material throughout its impingement area. One way of accomplishing it is to cause the flow of the oscillatory gas from the gas-distributing system be substantially equally split and impinged onto the surface of the material through a network of the discharge outlets. The apparatus of the present invention is designed to discharge the oscillatory flow-reversing impingement air or gas onto the material to be dewatered or dried according to a pre-determined, and preferably controllable, pattern. A pattern of distribution of the multiple discharge outlets may vary. One beneficial pattern of distribution comprises a non-random staggered array.
The discharge outlets of the gas-distributing system may have a variety of shapes, including but not limited to: a round shape, generally rectangular shape, an oblong slit-like shape, etc. Each of the discharge outlets has an open area xe2x80x9cAxe2x80x9d and an equivalent diameter xe2x80x9cD.xe2x80x9d A resulting open area xe2x80x9cxcexa3Axe2x80x9d is a combined open area formed by all individual open areas of the discharge outlets together. An area of a portion of the material to be dewatered or dried impinged upon by the oscillatory flow-reversing impingement field at any moment of the continuous process is an impingement area xe2x80x9cE.xe2x80x9d
In a continuous process of the present invention, the material to be dewatered or dried is supported by the support traveling in the machine direction. In one embodiment a means for controlling the impingement distance may be provided, such as, for example, conventional manual mechanisms, as well as automated devices, for causing the outlets of the gas-distributing system and the support to move relative to each other, thereby changing the impingement distance. Prophetically, the impingement distance may be automatically adjustable in response to a signal from a control device, measuring at least one of the parameters of the dewatering process or one of the parameters of the material being dewatered or dried. Depending on the nature of the material being dewatered and its qualities, including moisture content, the impingement distance may vary from about 0.25 inches to about 24.0 inches. The impingement distance defines an impingement region, i. e., the region between the discharge outlet(s) and the support. In one embodiment, a ratio of the impingement distance Z to the equivalent diameter D of the discharge outlet (i. e., Z/D) is from about 1.0 to about 10.0. A ratio of the resulting open area xcexa3A to the impingement area E (i. e., xcexa3A/E) may be from 0.002 to 1.000.
In one embodiment, the gas-distributing system comprises at least one blow box. The blow box comprises a bottom plate having the plurality of the discharge outlets therethrough. The blow box may have a substantially planar bottom plate. Alternatively, the bottom plate of the blow box may have a non-planar or curved shape, such as, for example, a convex shape, or a concave shape. In one embodiment of the blow box, a generally convex bottom plate is formed by a plurality of sections. In another embodiment, the blow box terminates with the plate having a prolong, slit-like slot extending in the cross-machine direction relative to the movement of the material to be dewatered or dried.
An angled application of the oscillating flow-reversing air or gas may be beneficially used in the present invention. Angles formed between the general surface of the support (or a surface of the impingement area E of the material being dewatered) and the positive directions of the oscillating streams of air or gas through the discharge outlet may range from almost 0 degree to 90 degrees. These angles may be oriented in the machine direction, in the cross-machine direction, and in the direction intermediate the machine direction and the cross-machine direction.
A plurality of the gas distributing systems may be used across the width of the material being dewatered. This arrangement allows a greater flexibility in controlling the conditions of the dewatering process across the width of the material being dewatered. For example, such arrangement allows one to control the impingement distance individually for differential cross-machine directional portions of the material being dewatered. If desired, the individual gas-distributing systems may be distributed throughout the surface of the support in a non-random, for example, staggered-array, pattern.
The oscillatory field of the flow-reversing impingement gas may beneficially be used in combination with a steady-flow (non-oscillatory) impingement gas impinged onto the material being dewatered. One embodiment comprises sequentially-alternating application of the oscillatory flow-reversing gas and the steady-flow gas. One of or both the oscillatory gas and the steady-flow gas can comprise jet streams having the angled position relative to the support.
The support may include a variety of structures, for example, papermaking band or belt, wire or screen, a drying cylinder, etc. In one embodiment shown herein, the support travels in the machine direction at a transport velocity.
Using the process and the apparatus of the present invention one can simultaneously remove moisture from differential density portions of the material being dewatered. The dewatering characteristics of the oscillatory flow-reversing process is dependent to a significantly lesser degree upon the differences in density of the material being dewatered. Therefore, the process of the present invention effectively decouples the water-removal characteristics of the dewatering processxe2x80x94most importantly water-removal ratesxe2x80x94from the differences in the relative densities of the differential portions of the material being dewatered.
One of the applications of the process of the present invention is in combination with application of pressure generated by a vacuum source. The apparatus of the present invention may be beneficially used in combination with a vacuum apparatus, such as, for example, a vacuum pick-up shoe or a vacuum box, in which instance the support is preferably fluid-permeable. The vacuum apparatus can juxtaposed with the backside surface of the support, preferably in the area corresponding to the impingement region. The vacuum apparatus applies a vacuum pressure to the material being dewatered or dried, through the fluid-permeable support. In this instance, the oscillatory flow-reversing gas created by the pulse generator and the pressure created by the vacuum apparatus can beneficially work in cooperation, thereby significantly increasing the efficiency of the combined dewatering process.
Optionally, the apparatus of the present invention may have an auxiliary means for removing moisture from the impingement region, including the boundary layer. Such an auxiliary means may comprise a plurality of slots in fluid communication with an outside area having the atmospheric pressure. Alternatively or additionally, the auxiliary means may comprise a vacuum source, and at least one vacuum slot extending from the impingement region, and/or an area adjacent to the impingement region, to the vacuum source, thereby providing fluid communication therebetween.
The present invention is believed to provide high water-removal rates and low air flow requirements, that results in reduced capital costs. The present invention is also believed to enable a material to tolerate high temperatures due to pulsating flows and ensure a reduced thermal damage to the material being dewatered or dried.